Micromechanical sensors and methods of manufacturing same

ABSTRACT

A micromechanical sensor and, in particular, a silicon microphone, includes a movable membrane and a counter element in which perforation openings are formed, opposite to the movable membrane via a cavity. The perforation openings are formed by slots, the width of which maximally corresponds to double the spacing defined by the cavity between the membrane and the counter element.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of pending U.S. patent application Ser.No. 10/866,582 filed Jun. 11, 2004, now U.S. Pat. No. 7,190,038; whichis a continuation of International Application No. PCT/EP02/12783, filedNov. 14, 2002, which designated the United States, and claims priorityto German application no. 101 60 830.6 filed on Dec. 11, 2001, which areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to micromechanical sensors and methods ofmanufacturing same and, in particular, to so-called micromechanicalone-chip sensors for the manufacture of which a wafer is required.

BACKGROUND OF THE INVENTION

In micromechanical sensors, of which a silicon microphone is an example,there are often air-filled cavities having very small dimensions. In amicrophone, this is, for example, the air-filled sensor capacityconsisting of a sensitive membrane and a rigid counter electrode. Due tothe small air volume, the entrapped air applies a strong restoring forceon the sensor membrane. This rigidity of the entrapped air lowers thesensitivity of the sensor.

It is known to provide discharge possibilities for the air, whereinthis, in silicon microphones, takes place by perforation of the counterelectrode. By means of such a perforation, the air can escape from thecapacitor gap, i.e. the cavity between the sensitive membrane and therigid counter electrode.

A large number of microphones and micromechanically manufacturedmicrophones are known from the prior art.

Commercial electret microphones comprise geometries in an order ofmagnitude in which the rigidity of the air cushion is negligible. Thesemicrophones do not have the advantages of a temperature-stable siliconmicrophone when produced in large numbers.

In micromechanically manufactured microphones, such ones havingelectroplated counter electrodes are known in which the counterelectrode is finally electroplated on the microphone chip. As regardssuch microphones, reference is, for example, made to Kabir et al., Highsensitivity acoustic transducers with p+membranes and gold black-plate,Sensors and Actuators 78 (1999), pp. 138-142; and J. Bergqvist, J.Gobet, Capacitive Microphone with surface micromachined backplate usingelectroplating technology, Journal of Microelectromechanical Systems,Vol. 3, No. 2, 1994. In methods of manufacturing such microphones, thesize of holes in the counter electrode can be selected such that theacoustic resistance is very small and does not influence the microphonesensitivity. The complicated process of electroplating is, however, ofdisadvantage.

Two-chip microphones are also known from the prior art, in which themembrane and the counter electrode are manufactured on respectiveseparate wafers. The microphone capacity is then obtained by bonding thetwo wafers. As regards such a technology, reference is made to W.Kuhnel, Kapazitive Silizium-Mikrofone (Capacitive Silicon Microphones),series 10, Informatik/Kommunikationstechnik, No. 202,Fortschrittsberichte, VDI, VDI-Verlag, 1992, Dissertation; J. Bergqvist,Finite-element modelling and characterization of a silicon condensermicrophone with highly perforated backplate, Sensors and Actuators 39(1993), pp. 191-200; and T. Bourouina et al., A new condenser microphonewith a p⁺ silicon membrane, Sensors and Actuators A, 1992, pp. 149-152.It is, as far as technology is concerned, also possible in this type ofmicrophone to choose adequately large diameters for the holes in thecounter electrode. For reasons of cost, however, one-chip solutions arepreferred. In addition, the calibration of the two wafers is problematicin two-chip microphones.

In one-chip microphones mentioned before, the counter electrode ismanufactured in an integrated way, i.e. only one wafer is required. Thecounter electrode is made of a silicon substrate and is formed by meansof deposition or epitaxy. Examples of such one-chip microphones aredescribed in Kovacs et al., Fabrication of singe-chip polysiliconcondenser structures for microphone applications, J. Micromech.Microeng. 5 (1995), pp. 86-90; and Füldner et al., Silicon microphonewith high sensitivity diaphragm using SOI substrate, ProceedingsEurosensors XIV, 1999, pp. 217-220. In the manufacturing processes forthese one-chip microphones, it is required or of advantage to close theholes in the counter electrode again for the following processing inorder to smooth the topology. In the well-known micromechanicallymachined microphones described above, the perforation openings in thecounter electrodes have a squared or circular cross-sectional form.

A manufacturing process for such one-chip microphones is known from WO00/09440. In this method for manufacturing, the perforation openings areformed at first in an epitaxial layer formed on a wafer. Subsequently,an oxide deposition on the front side of the epitaxy layer is performedso that the perforation openings are closed on the one hand and aspacing layer, the thickness of which defines the future gap between themembrane and the counter electrode is formed on the other hand. Asilicon membrane having the required thickness is then deposited on thislayer. After the required processing of the electronic elements, thewafer is etched from the backside down to the epitaxy layers in theregion of the perforation openings. Subsequently, etching of the oxidefrom the backside takes place for opening the perforation openings andthe cavity between the membrane and the counter electrode. A part of thesacrificial layer between the membrane and the epitaxy layer thusremains as a spacing layer between the membrane and the counterelectrode.

A method of manufacturing a one-chip microphone is known from DE19741046 C1, in which the counter electrode is patterned so to speak ina final manufacturing step after producing the membrane. Thus, it ispossible in this method to produce holes having a diameter of about 25μm to 50 μm or squares having an edge length of about 25 μm asperforation openings. In addition, this text teaches providingperforation openings in the counter electrode which have the form of arectangle which, with its longitudinal sides, extends over almost theentire edge length of the squared counter electrode and the width ofwhich corresponds to the edge length of the squares indicated above.

Finally, capacitive transducers are known from U.S. Pat. No. 5,870,482,in which border regions of a mounted membrane, together with a counterelectrode, serve as capacitive receptors. In one example, a roundmembrane mounted in its middle section is provided, while the outerborder region, together with a counter electrode spaced apart between 1μm and 4 μm, forms a capacitor. 14 μm slots having a spacing of 24 μmare provided in the counter electrode.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a highly sensitivemicromechanical sensor which can be manufactured in a flexible way, aswell as a method of manufacturing such a sensor.

In accordance with a first aspect, the present invention provides amicromechanical sensor having a movable membrane and a counter elementin which perforation openings are formed, opposite the movable membranevia a cavity, wherein the perforation openings are formed by slots, thewidth of which maximally corresponds to double the spacing defined bythe cavity between the membrane and the counter element.

In accordance with a second aspect, the present invention provides amethod of manufacturing a micromechanical sensor, having the followingsteps: producing slot-formed perforation openings in a layer; producingan essentially planar oxide layer on the layer in which the perforationopenings are formed, with a thickness corresponding to the futurespacing between a counter element and a membrane of the micromechanicalsensor, using a front-side oxide deposition on the layer provided withthe perforation openings, wherein the width of the slot-formedperforation openings maximally corresponds to double the thickness ofthe oxide layer; depositing the membrane on the essentially planar oxidelayer; and performing of etching for opening the perforation openingsand for producing a cavity in the oxide layer between the membrane andthe counter element in which the perforation openings are formed.

The present invention is based on the finding that the sensitivity ofmicromechanical sensors depends on the perforation density and on thesize of the perforation openings formed in an element adjacent to asensor cavity. In particular, the resulting acoustic resistancedetermines the upper cut-off frequency of the microphone sensitivity ina micromechanical microphone depending on the perforation density andthe size of the individual holes.

The present invention includes a novel perforation having slot-formedholes which are preferably arranged uniformly over the counter element,in the preferred embodiment of a silicon microphone, over the counterelectrode. According to the invention, the slots have a width whichmaximally corresponds to double the spacing defined by the cavitybetween the membrane and the counter element so that the processing ofthis perforation can easily be integrated into any overall process.Slots having such a width can be closed easily when manufacturing theinventive sensor by depositing the layer which is etched later forproducing the cavity since they grow to be closed from both sides whendepositing so that a safe closing of the perforation openings resultswith the size ratios given.

Such a maximal width of the slots, preferably together with a limitedlength of them, also ensures a sufficient stability of the counterelement or the counter electrode. In addition, a defined rigidity of thecounter element or the counter electrode can be ensured when the slotsare formed in at least two different directions. The slots arranged indifferent directions are thus preferably distributed alternatingly anduniformly over the counter element. According to the invention, theslots or slot groups are arranged in respective rows and columns in thecounter element, wherein the rows and columns are offset compared to oneanother. The acoustic friction resistance of the slot-formed perforationis considerably smaller than in the conventional round or squared holeswith an equal effective degree of opening.

According to the invention, longitudinal trenches instead of round orsquared holes are used for perforating. With an equal area density ofthe perforation, the viscous flow resistance of air is reduced by theperforation so that the area of usage of the respective sensor, such as,for example, the bandwidth of the microphone, is increased. Apart fromthe microphones described in the following as preferred embodiments, thepresent invention is also applicable to different micromechanicalsensors having a cavity arranged between a counter element and amembrane, wherein only acceleration sensors, force sensors and the likeare to be mentioned as examples. In addition, the present invention isnot limited to such sensors in which a capacitive detection takes place,but is also applicable to sensors utilizing a piezo-electric effect orthe like for producing a sensor signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be detailedsubsequently referring to the appendage drawings, in which:

FIG. 1 shows a schematic cross-sectional view of an inventivemicromechanical sensor;

FIGS. 2 a and 2 b are schematic illustrations for explaining the flowresistance through perforations openings;

FIGS. 3 a and 3 b are schematic illustrations showing an embodiment ofperforation openings provided according to the invention;

FIG. 4 is a diagram showing a comparison of the microphone sensitivityof the an inventive microphone and that of a well-known microphone;

FIGS. 5, 6 a and 6 b are schematic illustrations of perforation openingsaccording to further embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the general setup of an inventive siliconmicrophone is to be described at first, wherein such a microphone can bemanufactured with advantage using the method described in WO 00/09440.

As can be seen from the schematic cross-sectional view of FIG. 1, theone-chip silicon microphone comprises a movable membrane 10 made of ann⁺-doped polysilicon. The membrane 10, via a sensor cavity 12, isopposite to a counter electrode 14 formed by areas of an epitaxy layer15 formed on a substrate 16. A p⁺-doping area 18 and perforationopenings 20 are formed in the counter electrode 14.

The membrane 10 is attached to the counter electrode via a spacing layer22 which in the embodiment shown consists of oxide. A first terminalelectrode 24 is connected to the membrane 10 in an electricallyconductive way, while a second terminal electrode 26 is connected to thedoping area 18 of the counter electrode 14 in an electrically conductiveway. For this purpose, an insulating layer 28 is usually provided.

An opening 30 is provided in the substrate 16 below the section of theepitaxy layer 15 serving as the counter electrode 14 so that theperforation openings 20 connect the cavity 12 to the substrate opening30 fluidically. The opening 30 can be etched into the substrate 16,wherein in FIG. 1 a layer 32 which can serve as a masking layer for suchetching of the opening 30 is shown schematically.

Since the mode of operation of the sensor shown in FIG. 1 is obvious forthose skilled in the art, it is only mentioned briefly that a deflectionof the membrane 10 takes place by acoustic waves impinging on it so thata capacity change which takes place due to the changed distance betweenthe membrane 10 and the counter electrode 14, can be detected betweenthe terminal electrodes 24 and 26.

In order to reduce the influence of air in the cavity 12 on thesensitivity and the response behavior of the sensor, the perforationopenings 20 serving as discharge openings are provided in the counterelectrode. Air can escape from the capacitor gap, i.e. into the cavity12, through these perforation openings 20, wherein the resultingacoustic resistance determines the upper cut-off frequency of themicrophone sensitivity depending on the perforation density and the sizeof the individual holes.

A portion of the membrane 10 in a deflected state is shown schematicallyin FIG. 2, wherein only a portion of the counter electrode 14 isillustrated. By the deflection of the membrane 10, air between themembrane 10 and the counter electrode 14 is displaced and forced throughthe perforation openings 20. The perforation openings 20 thus act like aparallel connection of acoustic resistors, as is shown in FIG. 2 b. Theresistors 34 shown in FIG. 2 b with large dimensions thus represent theacoustic resistance produced by a respective perforation opening itself,while the resistors 36 shown with small dimensions are due to the flowresistance towards the respective perforation openings. The entire flowresistance thus is composed of a portion parallel to the counterelectrode, i.e. the flow towards the perforation openings, and a portionthrough the perforation openings. The flow resistance is thus defined bythe width, length and height of the perforation openings as well as bythe respective arrangement and density of the perforation openings.

The present invention provides micromechanical sensors in which theentire flow resistance and thus the acoustic resistance can be decreasedconsiderably. Such a decrease cannot be obtained by simply increasingthe perforation density at will since the static counter electrode mustbe sufficiently rigid. In addition, the flow resistance cannot bedecreased at will by arbitrarily increasing the cross-section or edgelength of well-known round or square perforation openings since suchperforation openings in this case can no longer be closed by adeposition of layer 22 with a height corresponding to that of the cavityso that such microphones can only be manufactured under certaincircumstances or with increased expenditure. With round or squared holesof the size which can be realized, the high acoustic resistance of theviscous pipe flow thus causes a decrease of the microphone sensitivitywith high frequencies.

The present invention makes possible a considerable decrease of the flowresistance compared to well-known round or square perforation openingsby providing slot-formed perforation openings. In order to make aclosing of the perforation opening possible for the following processingin order to smooth the topology, the slot-formed openings, i.e. thetrenches, according to an aspect of the present invention comprise awidth of maximally double the spacing defined by the cavity between themembrane and the counter element. In order to keep layer tensions in theoxide layer by means of which the perforation openings are closedlimited and to prevent the fact that thicker oxide layers than requiredmust be deposited, the slots according to the invention preferably havea width of at most 2 μm.

The length of the inventive slots or trenches can be selected to beconsiderably larger, wherein the length of the slots can, for example,correspond to double or triple the slot width. In any case, it ispreferred for the trench length to be only a fraction of the dimensionsof the exposed counter electrode in order to ensure a sufficientstability of the counter electrode. In any case, the length of theindividual slots preferably is less than half the dimension of thecounter electrode.

It can be shown by means of equations modelling the viscous, laminarpipe or gap flow that the acoustic resistance of a trench, with aconstant cross-sectional area, is considerably smaller than with a roundor squared hole. Thus, short narrow trenches allow a densely packedarrangement with a small inflow resistance with a simultaneous highstability of the counter electrode.

A preferred embodiment for the arrangement of such slot-formedperforation openings is a periodic and offset arrangement of relativelyshort and narrow trenches, ensuring a strongly decreased flow resistanceon the one hand and a high stability of the counter electrode on theother hand. Such an arrangement is shown in FIGS. 3 a and 3 b. The slots20 in this embodiment are arranged in two directions perpendicular toeach other, i.e. longitudinally and crossways and alternatingly in eachof the two directions. FIG. 3 a shows a top-view of such slots, while aschematic perspective illustration with a section through the counterelectrode is shown in FIG. 3 b. The slots 20 shown in FIGS. 3 a and 3 bcan, for example, have a width of 1.5 to 2.5 μm and a length of 5 to 10μm, wherein the width in one preferred embodiment essentially is 2 μm,while the length essentially is 6 μm.

In FIG. 4, curve 40 shows the microsensitivity of a microphonecomprising the offset structure shown in FIG. 3 a of perforationtrenches in the counter electrode of it, compared to the frequency,while curve 42 illustrates the sensitivity of a conventional microphonehaving round perforation openings. In both microphones, the perforationarea is 25% of the entire area. The hole diameter or the trench widthfor establishing the curves 40 and 42 was 2 μm. As can be easily seenfrom the microsensitivity illustrated in FIG. 4 compared to thefrequency, the upper cut-off frequency is more than doubled from 5.7 kHzto 13.2 kHz by the inventive perforation using trenches.

A further embodiment of inventive perforation openings is shown in FIG.5. In this figure, two respective perforation slots arranged indirections which are perpendicular to each other cross and form slotcrosses 50. The slot crosses 50 are arranged in rows, wherein the slotcrosses of neighboring rows are offset by one another and each arrangedin the middle between two neighboring slot crosses. Since the distanceof the trench walls is larger in the middle of each slot cross 50, it isof advantage to make the trench width slightly smaller than in the casethat the slots do not cross in order to make a closing of the slotcrosses with sacrificial oxide possible.

According to the invention, the slot-formed perforation openings aredistributed as evenly as possible over the area of the counterelectrode, wherein the offset arrangement described is of advantage forthis purpose. The periodicity of the regular grating is predetermined bythe area proportion of the perforation compared to the overall area,wherein the area proportion selected must ensure a sufficiently rigidcounter electrode with an active capacitor area. The offset arrangementfurther results in a dense arrangement of the perforation trenches sothat the flow resistance of air from the gaps to the perforationopenings is minimized.

Apart from the embodiments described, it is possible to arrange theslots arranged in different directions with another angle than 90°between them, wherein the advantage of an increased stability of thecounter electrodes can still be realized. Preferably, the slots arrangedin different directions are disposed in an alternating way to oneanother, wherein it is possible to arrange slots in more than twodifferent directions.

An alternative arrangement of perforation slots is shown in FIG. 6 a,wherein the perforation slots are arranged in the same direction inrespective rows and columns without any offset between them. In FIG. 6b, another alternative arrangement is shown, in which the perforationslots are arranged in the same direction in rows and columns, whereinthe slots of neighboring columns are, however, offset to one another.

According to the invention, both the small slot width and thearrangement of the perforation slots in different directions contributeto an increased stability of the electrode compared to well-knownsensors. Another increase in stability can be obtained by the describedperiodical and offset arrangement of relatively short narrow trenches.The narrow trenches also make possible using any method formanufacturing in which the sensor membrane is manufactured afterproducing the perforation openings.

A preferred embodiment of an inventive method for manufacturing themicromechanical sensor shown in FIG. 1 will now be discussed briefly. Inthis method, the slot-formed perforation openings 20 are at first formedin the epitaxy layer 15 which, preferably, has already been providedwith the p⁺-doped area 18, while the opening 13 is not yet provided inthe substrate 16. After producing the slot-formed perforation openings20, the oxide layer 22 is deposited in the required thickness, whereinthe perforation openings are thus closed. The perforation openings 20thus comprise such a width which enables the openings to be closed whendepositing the oxide layer 22 with the thickness as determined and whichfurther results in an essentially planar surface of the oxide layer 22deposited. Subsequently, the silicon membrane 10 can be formed directlyon the oxide layer 22.

Subsequently, the front side of the resulting structure is processed,for example to form the terminals 24 and 26, wherein electronic elementsare preferably manufactured in a conventional way in neighboring areasof the epitaxy layer. In a final step, the substrate 16 is opened fromthe backside using a corresponding mask layer 32 in order to expose theperforation openings closed with the oxide. Subsequently, selectiveetching of the oxide in the perforation openings 20 and of the oxidelayer 22 takes place in a conventional way to produce the cavity 12. Asregards such a method for manufacturing a micromechanical sensor, withthe exception of the perforation openings formed as slots, reference ismade to WO 00/09440.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents whichfall within the scope of this invention. It should also be noted thatthere are many alternative ways of implementing the methods andcompositions of the present invention. It is therefore intended that thefollowing appended claims be interpreted as including all suchalterations, permutations, and equivalents as fall within the truespirit and scope of the present invention.

1. A method of manufacturing a micromechanical sensor, comprising thefollowing steps: producing slot-shaped perforation openings in a layer;producing an essentially planar oxide layer on the layer in which theslot-shaped perforation openings are formed, with a thicknesscorresponding to a future distance between a counter element and amembrane of the micromechanical sensor, using a front-side oxidedeposition on the layer provided with the slot-shaped perforationopenings, wherein a width of the slot-shaped perforation openingsmaximally corresponds to double the thickness of the essentially planaroxide layer, and wherein the slot-shaped perforation openings are closedby oxide when producing the essentially planar oxide layer, therebyfacilitating an essentially planar top surface of the essentially planaroxide layer that opposes the layer having the slot-shaped perforationopenings; depositing the membrane onto the essentially planar topsurface of the essentially planar oxide layer; and performing an etchingfor opening the slot-shaped perforation openings in the layer byremoving any oxide from the essentially planar oxide layer that hadclosed the slot-shaped perforation openings and for producing a cavityin the essentially planar oxide layer between the membrane and thecounter element which comprises the layer in which the slot-shapedperforation openings are formed, wherein the length of the slot-shapedperforation openings is at least double the width of the slot-shapedperforation openings but less than half a dimension of the counterelement.
 2. The method according to claim 1, wherein the width of theslot-shaped perforation openings is 2 μm or less.
 3. The methodaccording to claim 1, wherein the slot-shaped perforation openings arearranged in rows and columns.
 4. The method according to claim 3,wherein the slot-shaped perforation openings are formed by first slotsand second slots which are perpendicular to the first slots.
 5. Themethod according to claim 4, wherein the first and second slots arearranged alternatingly in the counter element.
 6. The method accordingto claim 1, wherein the slot-shaped perforation openings are slotsformed in different directions.
 7. The method according to claim 1,wherein the slot-shaped perforation openings form slot crosses.
 8. Themethod according to claim 1, wherein slot-shaped perforation openingsare arranged in rows and the slot crosses of neighboring rows are offsetto one another.
 9. The method according to claim 1, wherein the area ofthe slot-shaped perforation openings basically is 10 to 50% of theentire area of the counter element.